Scanning laser ophthalmoscopes such as shown in U.S. Pat. No. Webb 4,765,730; U.S. Pat. No. Webb 4,764,006 and U.S. Pat. No. Webb 4,768,873 are known to include a turning mirror to direct a laser beam to a multi-faceted rotating polygonal reflector scanner that scans a laser beam in a first direction to form a line of light. A second scanner is employed in the form of a galvanometer reflector scanner to scan the line of light generated by the first scanner in a second direction perpendicular to the first direction of scanning. The scanned light is directed to a patient's eye by a series of focusing mirrors. Light reflected from the patient's eye follows the same path via the scanners and focusing mirrors back to the turning mirror. The turning mirror is small so that the light reflected from the eye passes around it to an optical detector in the form of an avalanche diode. The output of the optical detector is coupled to the display to provide a two dimensional picture of the patient's retina. Although this type of scanning laser ophthalmoscope is capable of producing an image of the patient's retina without requiring the patient's pupil to be dilated with drugs and without requiring contact with the patient's eye, it has several drawbacks. First, the scanned laser light source employed in Webb's scanning laser ophthalmoscope is very bright and leaves the patient dazzled for some time following the diagnostic procedure implemented with the ophthalmoscope. Further, although the choice of apertures and their location as well as the placement of the second scanning mirror in the Webb system produces a device with few unwanted reflections, these system elements do not allow ocular tissues to be viewed at varying depths. The Webb system also suffers from complexity, high cost and provides a small field of view that is on the order of only 30.degree..
Another type of scanning laser ophthalmoscope is shown in U.S. Pat. No. Kobayashi 4,781,453 that utilizes a first acousto optical modulator for modulating the intensity of a laser beam to project a fixation target. The frequency of the drive signal for the first acousto optical modulator is also varied so as to select, with the use of a lens and device having a slit therein, a single wavelength of a laser beam having a number of wavelengths therein. The single selected wavelength of the laser is then passed to a scanning system. The scanning system includes a second acousto optical modulator that is driven so as to scan the selected wavelength of the laser in a first direction. Prior to scanning, however, the range of the second acousto optical modulator must be changed to accommodate the selected wavelength of the laser. The scanned laser is guided by relay lenses from the second acousto optical modulator to a mirror that is mounted on a galvanometer for scanning the laser in a second direction perpendicular to the scanning direction of the second acousto optical modulator. A small mirror then reflects the scanned light to a patient's eye. The light reflected from the eye passes around the small mirror and is captured by a lens and focused on a photosensor. A filter corresponding to the selected wavelength of the laser is disposed in front of the photosensor to allow passage of the selected light to the sensor. An image of the eye at a depth corresponding to the selected wavelength is stored in a frame memory associated with the selected wavelength, wherein the system includes different frame memories for the different wavelengths that can be selected. The different images stored in the frame memories can be selected via the electronics of the system for individual display in different colors. The Kobayashi ophthalmoscope is an extremely complex device in which the scanning range of the second acousto optical modulator must be changed to accommodate a selected wavelength of the laser light each time a new wavelength is selected via the first acousto optical modulator. Further, the filter disposed in front of the photo sensor must also be changed in accordance with the selected wavelength. The field of view of this scanning laser ophthalmoscope is also small, being on the same order as that described above for the Webb scanning laser ophthalmoscope.
Further, in both the Webb and Kobayashi systems, a mirror is disposed in the optical path of the light reflected from the patient's eye to the detector which causes a shading off effect. This shading off effect is realized as a darkening of the edges of an image feature with a gradual lightening of the image feature towards the center thereof. For example, this effect causes the displayed image of a blood vessel to appear as dark parallel lines with a lighter center therebetween. This effect is further exacerbated by the small aperture diameter employed in the image detection portion of the these systems. This small aperture although eliminating unwanted reflections from detection, brings substantially all of a given scene into focus at the same focal plane. The result is that the image of the patient's fundus appears similar regardless of the wavelength of the laser beam and the portion of the patient's eye at a particular depth therein reflecting the selected wavelength of the light.